1. Field of the Invention
The field of art to which this invention relates is an apparatus for semi-active isolation of payloads with low transmissibility, and more particularly to an apparatus having semi-active means for isolating vibration of a payload from a base structure.
2. Description of the Related Art
Traditionally, one of two approaches have been used to isolate a payload from the vibrations of a base structure. Either the payload is very rigidly connected, to the base structure such that it could withstand the effects of the vibrations or the payload is connected to the base structure by vibration damping elements such that the payload is free to move relative to the base structure.
In certain applications, the former approach is impractical because of the extra material and weight needed to make the very rigid connection between the payload and base structure. Launch vehicles are one such application. The heavier a launch vehicle is, the more thrust is needed to lift it to its predetermined destination. Added thrust means bigger, heavier, and more expensive engines which transmit even more vibrations to the payload.
The latter approach also has its disadvantages in certain applications, such as launch vehicles. Referring to FIG. 19, there is illustrated a schematical representation of a conventional launch vehicle 400 having a payload 101, a base structure 102, engines 403 fixed to the base structure 102, damping elements 404 disposed between the base structure 102 and the payload 101, and a fairing 405 covering the payload 101 and base structure 102. The payload 101 generally houses sensitive electronics and other sensitive equipment that is prone to failure due to vibrations that can be transmitted to the payload 101 through the base structure 102 by the engines 403. As the launch vehicle 400 travels, the effective weight of the payload 101 varies due to the acceleration of the launch vehicle 400 as well as changing mass of the launch vehicle 400 (due to the consumption of fuel which powers the engines 403).
The damping elements 404 generally require a relative movement between payload 101 and the base structure 102 in the vertical (axial) and horizontal (lateral) directions. Unfortunately, these damping elements also allow some rotation of the payload 101 with respect to the base structure 102. Since the height of the launch vehicle payload is typically great, even a slight rotation of the payload results in a large displacement near the top of the payload 101. However, the fairing 405 of the launch vehicle 400 typically only has approximately one inch of clearance between its inner surface and the outer surface of the payload 101. Thus, to avoid contact between the fairing 405 and the payload 101 even the slightest rotation cannot be tolerated.
To counter this problem, heavy, active rotational restraint systems are necessary, typically comprising at least three voice coil motors which direct a restoring force to the payload to keep it from rotating. A feedback system senses the rotation of the payload 101 and signals the motors to direct force accordingly. Such an active system is heavy, complicated, and prone to failure. Furthermore, such an active system requires electrical power to drive the motors which is either siphoned from the engines or stored in heavy batteries. Neither of which is very desirable in a launch vehicle.
For the above reasons, there is a need in the art for a payload isolation system which is low weight, uncomplicated, does not permit rotation of the payload relative to the base structure, and preferably mechanical which operates in a passive or semi-active mode so as not to be prone to failure or require an undesirable energy drain.
Therefore, it is an object of the present invention to provide an apparatus for isolating a payload from a base structure upon which it is supported so as to suppress the transmission of vertical (axial) and/or horizontal (lateral) vibration between the payload and base structure.
It is a further object of the present invention to provide an apparatus for maintaining effective isolation in the presence of varying effective payload weight, which may be due to either variation of the vertical acceleration field or of the payload mass, or both.
It is yet a further object of the present invention to provide an apparatus for maintaining the natural frequency of vertical isolation means substantially constant and low while accommodating the variations in the effective weight with substantially no variations in the deflection.
It is yet a further object of the present invention to provide an apparatus for providing an automatic or convenient manual means to adjust the isolation for variations in the effective payload weight and optimize for lowest practical natural frequency.
It is yet a further object of the present invention to provide an apparatus for providing the desired isolation entirely passively, i.e., with no expenditure of energy, during periods when the payload is substantially fixed and when the payload varies, by using active means solely to adjust the parameters of the passive system to compensate for payload variation, i.e., by constructing a semi-active means of control.
It is yet a further object of the present invention to provide an apparatus for preventing the transmission of vertical and lateral vibration forces, prevent any relative rotational motion between the base and the payload.
It is still yet-a further object of the present invention to provide an apparatus for preventing excessive buildup of destructive resonant oscillations if the natural frequency of the isolation system should overlap with the spectrum of vibrational excitation to which it is exposed.
The present invention provides a novel means for passively isolating a payload from a base structure, upon which it is supported, to prevent transmission of both vertical (axial) and lateral vibration between. the payload and base structure, while providing for active parameter adjustment to compensate for changes in the payload mass or effective weight or the required isolation spectrum with very low cost in terms of power consumption, weight and volume. Here, lateral is defined for the purposes of the invention as being orthogonal to the quasi-static acceleration field, i.e., the vertical (axial) direction. The quasi-static acceleration in this context may be, for example, gravity or the average gross acceleration of a vehicle carrying the payload. The acceleration is considered to be quasi-static when its variation is slow compared with the frequency of structural vibrations being isolated.
With the present invention, vertical and lateral transmissibility of vibration is substantially reduced above a relatively low system natural frequency, such as above about 5-10 Hz, which requires a relatively low effective isolation system spring rate for a given payload mass, while the static or quasi-static spring deflections are simultaneously small. With this invention, such a capability is achieved with a nonlinear elastic element that is relatively rigid at low and at high levels of displacement, but is very compliant at intermediate levels of displacement.
The present invention also provides an improved means for making the system rigid to rotational deflections while simultaneously providing the extreme flexibility to vertical and lateral displacement that is necessary to achieve the low system natural frequency that is required for the desired low vertical and lateral vibration force transmissibility. This is achieved by mechanically constraining the rotational motion of the payload relative to the base structure, with variations of properly placed one or more parallelogram linkages.
Moreover, the present invention also provides improved means for preventing buildup of resonant oscillations when the system is subject to vibrational excitation at frequency near its natural frequency. The improved means utilizes nonlinear elasticity such that the natural frequency of the system in the vertical direction and if desired in the lateral directions stays nearly constant at only small amplitudes of vibrational displacements. At larger amplitudes of vibrational displacements, due to the nonlinearity of the spring rate, its effective spring rate changes, thereby shifting the natural frequency of the system, thereby effectively preventing resonance without degradation in the performance of the isolation system.
A typical payload isolation system constructed in accordance with the present invention consists of several functional elements including motion constraint means, vertical load support means and adjustment means, active driver means, accessories that may be required if the system is to be entirely self powered and self contained, and elastic means for lateral isolation. In an integrated design, it is possible and may sometimes be efficient that two or more functional means as defined above will be represented in a single physical component.
In a system according to the present invention, the motion constraint means will preferably be a structural arrangement consisting of one or more parallelogram linkages that have the function of preventing the payload from rotating relative to the base structure. In addition, this element may also be used for the support of the quasi-static vertical load, or control of vertical and/or lateral vibration, by utilizing such linkages in complementary pairs or by restraining the motion of certain links elastically. In the latter situation, the parallelogram linkage may be made to bear part or all the structural loads that result from such restraint.
Moreover the same system can also be made flexible with respect to and isolate, vibrations that are rotational about the vertical axis. In general, this does not require additional functional components or types of components but is a function of the details of the isolation design, particularly the flexibility of the links of the parallelogram linkages in the directions that contribute to the aforementioned rotation.
The vertical load support means is the component or set of components that supports the primary quasi-static effective weight of the payload. It preferably includes nonlinearity in its force-deflection characteristic such that the primary quasi-static load is borne with relatively little deflection at an effective operating point. The second function of the said force-deflection nonlinearity is to provide relatively large deflections in the presence of small variations in the effective load due to the low effective dynamic spring rate at the operating point. The third function of the said force-deflection nonlinearity is to provide the support means with the capability to bear more substantial increases in the effective load with relatively small additional deflections due to substantial rises in the effective spring rate for deflections higher than and outside the vicinity of the operating point. The effective spring rate of the system in the vicinity of the operating point is preferably low enough to permit a relatively low natural frequency of vibration, e.g., in the range of 5 to 10 Hz in concert with the payload mass. It is also possible that the vertical load support means may be comprised of several components acting in parallel and in series, such as a linear spring in series with a structural component exhibiting a nonlinear elastic behavior, i.e., force-deflection characteristics.
In one preferred embodiment of the invention, the vertical load support means is comprised of an array of elastomeric structures that xe2x80x9cbucklexe2x80x9d under load. It is preferable that the formulation of elastomeric material of which these structures are made be selected to exhibit the lowest possible damping characteristics. The array of structures, which would be designed and proportioned to xe2x80x9cbucklexe2x80x9d at a threshold load, may be in a variety of forms such as, for example, molded arches, or tubes each of which represents two symmetrically opposed semi-circular or appropriately designed curved arches, or vertical columns, or pairs of symmetrically non-vertical columns.
Each of these geometries will impart different properties to the support means, particularly in regard to non-vertical elastic characteristics and means of adjustment. However, all the variations share in common the feature that they can be designed to exhibit the desired nonlinearity in the vertical force-deflection characteristic.
One of the advantages of the preferred embodiment as buckling elastomeric structures is the degree of design flexibility offered by this class of structures. The spring rate in the vicinity of the operating point can be made to be very small and even zero or negative, as compared to the spring rate for large deflections, as desired to satisfy the requirements of a specific application.
The desired nonlinearity in elastic characteristics can also be approximated, for example, with one or more pneumatic or nominally linear springs held in preload at threshold levels of deflection, or by applying force through a nonlinear linkage such that the mechanical advantage through which a spring applies force increases or decreases with deflection in a predetermined way to compensate for and effectively modify the linear spring rate. Moreover, any combination of such methods may be used to obtain the desired characteristic and adjustment capability most economically in the context of the requirements of an application.
Preferably, the vertical load support means is adjustable so that the quasi-static load of effective payload weight may be varied over a substantial range with little or no change in the quasi-static deflection of the system about its operating point. In one preferred embodiment of the invention, the adjustment means is inherent in the construction of the vertical support means which acts as a pneumatic actuator and provides the desired load support adjustment with gas pressure, then its load bearing capability in any position is simply proportional to the amount of gas and gas pressure with which it is filled. The vertical load support means may also be provided by an appropriate external mechanism, such as for example a linkage or cam, to effect the adjustment of its load support properties. This external mechanism is what is referred to as the load support adjustment means. Several examples of such mechanisms are described below and in the accompanying figures.
This load support adjustment means is driven by active driver means, which is a transducer that provides the mechanical power and actuation to perform the adjustment. The driver means, which is preferably the only powered element in the system and is what keeps vertical deflection substantially constant in the presence of a varying effective payload weight, is preferably pneumatic but may also be, for example, electrically powered.
Finally, accessories may be required to perform certain functions if the system is to operate in a completely self-contained fashion. For example, a source of stored energy such as a compressed or liquefied gas, batteries or fuel, may be required if the system must operate independently of outside energy sources. Also, a sensor will be required to track deflection of the vertical load support element and provide a feedback signal which will be used to adjust the load support means as described above.
If the form of energy storage and the method of actuation is pneumatic, then it is preferable that the sensor used to track deflection of the payload should also be pneumatic and operate an appropriate control valve directly, rather than convert the sensed condition to a different kind of signal, such as electrical, and then back to pneumatic. However, the payload position relative to the base structure will be fluctuating rapidly due to the vibrations being isolated, and it is desired that the feedback signal to the control valve should represent only an average relative position over many cycles, varying at a slow rate comparable to that of the payload effective weight and quasi-static load variation. Consequently, it is preferred that the sensor and the control valve include low-pass filter means, preferably entirely mechanical in construction and operation, so that the control valve operation does not include a response to relatively high frequency deflections.
The above described functional means comprise the vertical load support and isolation components of the system. If the vibrational excitation does not have a lateral component and only vertical isolation is required, then additional means are not needed and the above described means may comprise the entire system. However, if significant lateral vibration is also present, then lateral isolation means may also be included to provide elastic control of motion in the directions orthogonal to the vertical direction.
The elastic characteristics of the lateral isolation means should preferably also be highly nonlinear. Indeed, its force-deflection curve should preferably resemble that of the vertical load support means, but differing in that it is displaced so that its operating point is at zero force and displacement. In this way, the effective lateral spring rate for small displacements is relatively low to provide for low natural frequency of the system and thereby for low transmissibility, but increases significantly at large displacements to effectively limit the range of motion of the payload relative to the base structure, as well as to provide the frequency shifting effectxe2x80x94similar to what was previously described for the vertical isolation meansxe2x80x94to prevent buildup of resonant oscillations.
It is possible that two or more of the functional means described may be combined in a single physical component of an integrated design. However, there may be very great disparities between the magnitudes of the quasi-static load and the vertical and lateral vibrational loads, so that the design of a single component intended to bear more than one of these loads, or to perform more than one related function, may not lead to the most efficient structure or system with respect to weight, volume, power consumption, or cost. Most generally, it is expected that the system may be comprised of components that are specialized to their functions.
Overall, the present invention provides improved means to isolate a payload from vibration, even at very low frequencies without sacrificing level control and stability of orientation, and without the risk of low frequency resonance. Also provided are means to adjust the isolation system, or even to make it automatically self-adjusting, so that it can accommodate variable effective payload weight with no change in its quasi-static position or deflection. Moreover, these benefits are provided with substantially lower weight, volume and power requirements than prior art systems directed to the same or similar objectives.
Accordingly, the payload isolation system of the present invention is provided for isolating a payload from a base structure upon which the payload is supported. In its basic configuration the payload isolation system comprises: motion constraint means for maintaining a parallel relationship between the payload and the base structure; and support means for providing vertical and/or lateral support of the payload relative to the base structure such that the transmission of vertical and/or lateral vibration between the payload and the base structure are suppressed.
In a preferred implementa the paylisolation system of the present invention, the motion constraint means comprises a mechanical linkage, such as a parallogram linkage and/or a scissor linkage. More preferably, at least two of such linkages are provided and at least two of the linkages are arranged non-parallel to each other.
In another preferred implementation of the payload isolation system of the present invention, the support means exhibits nonlinear elastic characteristics in response to an effective weight of the payload. Preferably, the non-linear elastic characteristics comprise a substantially rigid characteristic at low and high levels of deformation and a compliant characteristic at intermediate levels of deformation.
Also provided is a motion constraint mechanism. The motion constraint mechanism comprises: a first mechanical linkage disposed between a payload and a base structure; and at least a second mechanical linkage arranged relative to the first mechanical linkage such that the first and at least second mechanical linkages maintain a parallel relationship between the payload and the base structure. Preferably, the first and at least second mechanical linkages are arranged non-parallel to each other.
In a preferred implementation of the motion constraint mechanism of the present invention at least one of the first or at least second mechanical linkages comprises a parallelogram linkage disposed between the payload and base structure.
Preferably, each of the parallelogram linkages comprises first and second parallelogram sub-linkages. The first and second parallelogram sub-linkages sharing a common member. One of the first or second parallelogram sub-linkages is fixed to the payload or a portion thereof, the other of the first or second parallelogram sub-linkages is fixed to the base structure or a portion thereof.
In another preferred implementation of the motion constraint mechanism of the present invention, at least one of the first or at least second mechanical linkages comprises a scissor linkage having first and second scissor sub-linkages disposed between the payload and base structure. The first and second scissor sub-linkages are connected to each other by first and second common members. A first end of each of the first and second scissor sub-linkages is fixed to the payload or a portion thereof and a second end of the first and second scissor sub-linkages is fixed to the base structure or a portion thereof.
Also provided is a support apparatus for providing vertical and/or lateral support of a payload relative to the base structure such that the transmission of vertical and/or lateral vibration between the payload and the base structure are suppressed. The support apparatus comprises: a deformable member exhibiting nonlinear elastic characteristics in response to an effective payload weight; support adjustment means for supporting the effective payload weight; and effective payload adjustment means for adjusting the level of support of the support means in response to a varying effective payload weight. The deformable member preferably has at least one internal tubular cavity and more preferably a plurality of internal tubular cavities interconnected to each other such that the plurality of internal tubular cavities act as a single cavity.
In a preferred implementation of the support apparatus of the present invention, the effective payload adjustment means comprises feedback means for sensing a change in relative distance between the payload and the base structure and controlling the support adjustment means in response thereto.